Radiometry is a technology which involves the measurement of the amount energy which exists within a certain portion of an electromagnetic spectrum. Of particular interest is the measurement of the wavelengths from about 200 nanometers to 450 nanometers which are generally identified as the ultraviolet (UV) region. These wavelengths are used in numerous industrial processes over a very wide range of energy levels. Such measurements are challenging to make because optical component performance tends to be non-constant with wavelength. In addition, the measurements are susceptible to drift due to temperature, time, and variables often introduced during the data collection process in production/manufacturing environments or a lab environment. FIG. 1 shows the spectrum associated with a typical non-LED UV source.
The introduction of UV generating light-emitting diodes (LEDs) has produced a necessary change in UV energy measurement technology. UV LED radiometers measure the quantity of LED generated UV energy from a LED source. However, there have been numerous questions regarding the accuracy or, in some cases, the validity of the resulting measurements. The uncertainty is, in substantial part, a result of lack of formal definition of the portions of the spectrum being measured (spectral bands) and the lack of an instrument capable of making absolute energy measurements over specific bands of the defined spectrum. Most commercially available industrial radiometers describe their optical response in terms of their filter response only and not the total optical response of all optical components that energy passes through.
Generally, only a portion of the wavelengths in a given distribution are of interest. For example, in a curing process, the chemical reaction initiated by the ultraviolet energy occurs faster or slower according to the wavelength(s) of the UV energy impinged on it. The speed of the reaction is wavelength and irradiance dependent. A faster reaction, if it can produce the same desired results in the cured product, is generally better because it decreases processing time and, hence, reduces processing costs. Compromises are often made to achieve desired physical properties such as hardness, gloss, friction, color, and/or durability in the finished product. An ultimate objective is to be able to quantify the UV driven process so that the process can be documented and replicated.
FIG. 2 shows the optical absorbance curve of a typical photoinitiator. This can also be referred to as a curing sensitivity curve. The UV curing industry believes that optical absorbance of the photoinitiator is related to the ability of a given UV wavelength to generate free radicals in the photoinitiator and, hence, implement curing (polymerization). The exact relationship between absorbance and curing is not explicitly known, but it is generally agreed that different wavelengths have greater or lesser ability to implement curing than other wavelengths. It is this wavelength relationship that contributes to the need for UV radiometers to provide measurement of different and very specific wavelength ranges.
Different types of UV sources with different wavelengths are also used to achieve different properties in the product. For example, mercury-gallium bulbs are often used where depth of cure is needed, and mercury bulbs with enhanced shortwave UVC are used to achieve desired surface cure properties. As industrial manufacturers shift to using LED UV sources, there is a growing need for instruments and methods which can provide accurate measurements of non-LED and LED sources and permit their comparison in order to effectively transition equipment to LED UV sources.
Over the years, efforts have been made to measure the UV irradiance level and total UV energy impinged on a workpiece in order to implement proper curing. However, accurate and reproducible absolute measurements of energy and irradiance of UV sources has historically proved difficult if not impossible.
Radiometer manufacturers typically show only the spectral response of the bandpass filter alone and neglect optical response contributions by other components such as photodiode, diffuser, protective window, and attenuator. In the case of present day instruments, some of which use color interference bandpass filters, no provision is made for optical response changes caused by various factors including angle of incidence (AOI) factors. In the case of cut glass bandpass filters, angle of incidence factors are not as relevant. However, in both cut glass and color interference filter cases, the spectral response of the instrument is not rectangular. This is a shortcoming of existing instruments.
An instrument's response is substantially different from the filter response which manufacturers typically publish somewhat misleadingly as representing the overall optical response of their instrument(s). While this practice does not prevent the instruments from being used in a relative measurement mode, the results obtained can be much different from those obtained with a rectangular response and generally do not provide accurate irradiance and absolute energy measurements.